1. Field of the Invention
2. Description of the Prior Art
Media with long pores are of interest because of their capabilities to sort, direct and mechanically fix smallest chemical or biological units. Various attempts have been undertaken to incorporate porous silicon or porous alumina in microelectronic or microfluidics devices. A remaining problem is that up to now pure aluminum substrates are required and that in all cases anodized pore growth was perpendicular to the aluminum surface. If one were able to make lateral or directed pores in some material, they could be integrated into bio-analytical systems. Additional possible applications arise if pore growth could be directed or pores jointed together in a well-controlled way.
Porous media are also of interest because of their filtering capabilities. Various attempts have been undertaken to incorporate porous silicon or porous alumina in microelectronic or microfluidics devices. Again in regard to porous alumina, one remaining problem is that up to now pure aluminum substrates are required and that in all used cases pore growth was perpendicular to the aluminum surface. If one were able to have lateral or directed pores in some material, they could be integrated into cheap nanofluidic systems.
Much work has been done to explore the growth of vertical pores in alumina and filter membranes using vertical pores in alumina have been developed. All studies use unstructured anodization of pure aluminum sheets as a substrate, as its purity, compactness and polycrystalline arrangement are prerequisite for well-ordered pore growth. One publication reports anodization on a niobium-masked Al substrate and shows short pores that grow isotropically under a mask layer forming random branches under constant voltage and lifting the mask""s rim upward.
What is needed is to find a technical way to growing long pores underneath a mask, direct them, leave a vertical entrance direction and obtain well ordered, well-defined spreading or joining/branching pore structures.
What is further needed is a method to fabricate in-plane filters by opening the endings of lateral pores. The elements should be formed in a monolithic, compatible process and combined with fluidic inlets and outlets. Finally methods of integration these filters into planar fluidic systems are needed to make them suitable for use in cheap nanofluidic systems.
The invention is a method for forming a lateral pore in a film having an in-plane extent and a vertical direction perpendicular thereto comprising the steps of disposing a stress compliant mask on the film, defining a vertical hole through the stress compliant mask and into the film, and forming a lateral pore in the film by anodization.
The method further comprises the step of disposing a polymer layer on the stress compliant mask. The film has regions which will be porous and nonporous, which regions have at least one boundary between them. The stress compliant mask disposed on the film comprises is a planarized stress compliant mask disposed over or on the boundary of the film between of nonporous regions and porous regions. The step of disposing the stress compliant mask on the film comprises disposing on the film multiple composite mask layers. The multiple composite mask layers comprise at least a first layer bearing high intrinsic tensile stress and adjacent thereto at least a second layer bearing compressive stress. In one embodiment the multiple composite mask layers comprise at least a SiO2 layer and disposed adjacent thereto at least a Si layer. In another embodiment the multiple composite mask layers comprise at least a SiC layer and disposed adjacent thereto at least a Si layer. In still a further embodiment the multiple composite mask layers comprise at least a Si3N4 layer and disposed adjacent thereto at least a SiO2 layer. In any case the stress compliant mask on the film is a mechanically stable mask which withstands stress during anodization and counteracts pore formation stress to lead to pore ordering and directed growth. In other words the multilayer mask on the film has a composition of materials with different elastic properties such that tensile stress in the film is at least approximately matched to counteract compressive stress in the film caused by porous material growth. The disposition of the planarizing mask material provides locally increased masking layer thickness at the boundary between nonporous and porous material in the film.
The method further comprises annealing the film to improve its polycrystalline structure and prepare it for ordered pore growth, and/or disposing a passivating layer on the film to avoid oxidation during annealing.
The step of forming the lateral pore in the film by anodization comprises defining a start hole through the stress compliant mask and then anodizing the film through the start hole with an approximately constant anodizing voltage. In another embodiment the step of forming the lateral pore in the film by anodization comprises defining a start hole through the stress compliant mask with a nonrectangular geometry of a pore formed thereby and then anodizing the film through the start hole with a time varying anodizing voltage dependent on the nonrectangular geometry. When the nonrectangular geometry is trapezoidal, the anodizing voltage, V, is varied as determined by the equation dy/dx (V0v)/y0=dV/dt, where dy/dx is the change of width of the pore with respect to length of the pore, V0 is the starting anodizing voltage, v is the rate of pore growth, and y0 is the starting width of the pore. When the nonrectangular geometry is circular, the anodizing voltage, V, is varied as determined by the equation, dV/dt=xcfx80V0 v/y0, where V0 is the starting anodizing voltage, v is the rate of pore growth, and y0 is the starting diameter of the pore.
In another embodiment the step of disposing a stress compliant mask on the film comprises disposing a stress compliant mold on the film, or disposing a stress compliant mold on the stress compliant mask.
The method further comprises the step of removing the stress complaint mask including the vertical hole defined therein and all other structures adjacent to the lateral pore except for a wall a nonporous material adjacent to the lateral pore to create at least one lateral test tube. The test tube can be loaded with a microsample by electromigration. The microsample can be read, marked, modified or cut in the test tube by means of scanning electron microscopy. The microsample in the test tube can also be read by means of a near field optical microscope. An aperture can be defined in the test tube for disposition of a tip of an atomic force microscope therein and the microsample read, or modified in the test tube by means of atomic force microscopy.
The lateral pore has a first and second end and the method further comprises the steps of opening the first and second end of the pore, and disposing a wire in the pore. The wire has a first and second opposing end. The first and second opposing end of the wire is connected to electrical contacts. The first and second opposing end of the wire may be connected with the electrical contacts either by forming the electrical contacts adjacent to the first and second opposing end of the pore prior to the wire being disposed therein and contacting the first and second opposing end of the wire with the previously formed electrical contacts, or by forming the electrical contacts adjacent the first and second opposing end of the pore after to the wire is disposed therein and contacting the first and second opposing end of the wire with the subsequently formed electrical contacts.
The method further comprises forming at least two interconnected lateral pores in the film. In one embodiment the interconnected lateral pores in the film are formed by selectively disposing prior to anodization at least two interconnected nonporous channels of anodizable material in the film. In another embodiment the interconnected lateral pores are formed in the film by anodizing the pores in the film by a first electric field pattern and anodizing an interconnection between the at least two pores by a second electric field pattern. The first and second electric field patterns have differing field orientations with respect to each to cause the at least two pores defined by the first electric field pattern to interconnect when further formed by the second electric field pattern.
The method further comprises the steps of disposing a first linear sample into a first one of the at least two interconnected lateral pores in the film by driving the first linear sample therein in a first direction. A second linear sample is disposed into a second one redo of the at least two interconnected lateral pores in the film by driving the second linear sample therein in a first direction. The first and second linear samples are joined together to form a junction between them by driving the first and second linear samples is a second direction opposing the first direction so that the first and second linear samples are driven together.
A plurality of lateral pores are formed in a two dimensional array having a corresponding first array of pore openings and the method further comprises growing the two dimensional array of lateral pores into a corresponding spreading two dimensional array of lateral pores in the film by varying anodization voltage during growth such that a larger corresponding second array of pore openings is formed in communication with the first array of pore openings. In one embodiment the method further comprises filling the lateral pores communicating the first array of pore openings with the second array of pore openings with a conductive material to form a multiprobe stage. In another embodiment the method further comprises filling the lateral pores communicating the first array of pore openings with the second array of pore openings with a transparent material and coupling the pore openings of the second array with a near field optical microscope to form a nonscanning pixelated SNOM. The method further comprises using the lateral pores communicating the first array of pore openings with the second array of pore openings as fluidic channels to form a liquid probe array.
The method further comprises the step of defining another vertical hole through the stress compliant mask and into the film adjacent to an end of the lateral pore so that the lateral pore is opened at both opposing ends.
The method still further comprises simultaneously forming at least two collinear lateral pores in the film in opposing collinear arms of a tee-shaped film having a central arm extending perpendicularly from and between the two opposing collinear arms of the tee-shaped film, and applying an anodizing voltage to the central arm during further growth of the at least two collinear lateral pores into the central arm so that the collinear lateral pores interconnect with each other so that an open filter element is formed in the tee-shaped film.
In one embodiment the lateral pore has a closed backside and the method then further comprises opening the closed backside of the lateral pore by etching away nonporous material under the stress compliant mask adjacent to the closed backside to open the lateral pore at both ends. The method further comprising the step of disposing an etch-resistant layer beneath an anodizing electrode used in formation of the lateral pore.
In the embodiment where the stress compliant mask is an anodization hardmask and the method further comprises the step of removing a nonporous block under the anodization hardmask and then forming the lateral pore in nonporous material previously adjacent to the nonporous block so that a fluidic channel is formed laterally underneath the anodization hardmask.
In the embodiment where the stress compliant mask is an anodization hardmask and in one embodiment the method further comprises removing a sacrificial block under the anodization hardmask and then forming the lateral pore in material adjacent to the sacrificial block so that a fluidic channel is formed laterally underneath the anodization hardmask. The method further comprises disposing a molding over the anodization hardmask and fluidic channel.
In one embodiment the method further comprises the step of forming at least two lateral pores in the film by anodization at different times. The two lateral pores have correspondingly different diameters so that lateral filters of different sizes are formed.
The method further comprises forming at least two lateral pores in the film by anodization with different electric fields. The two lateral pores have correspondingly different diameters so that lateral filters of different sizes are formed. In one embodiment the method further comprises providing a voltage divider, forming at least two lateral pores in the film by anodization with different electric fields where the different electric fields provided by corresponding different electrodes adjacent to the film. The corresponding different electrodes are coupled to the voltage divider. The two lateral pores have correspondingly different diameters so that lateral filters of different sizes are formed. The method further comprises using the anodizing electrode and etch-resistant layer as a passivated electromigration electrode for the adjacent opened pore.
The method further comprises the steps of forming a plurality of the lateral pores to create a lateral filter in a fluidic channel within the film and vapor pumping liquid through the filter. The fluidic channel has at least one outlet port and the method further comprises the step of forming another plurality of the lateral pores to create a filter at the at least one outlet port so that an evaporator membrane is formed for the vapor pumping. In the illustrated embodiment the step of forming the plurality of the lateral pores to create a lateral filter in a fluidic channel and forming the other plurality of the lateral pores to create a filter at the at least one outlet port are performed simultaneously. The method further comprises providing an external flap disposed over the filter at the at least one outlet port for evaporation control.
The invention is also understood to comprise the apparatus which are formed by the foregoing methods of fabrication. In addition, although the invention has been described as be fabricated by a combination of steps, it is to be expressly understood that the claims of the invention are not to be construed as limited by the specification under 35 USC 112 but include all methodologies and structures falling within the definition of the claim wording. The invention can now be better visualized by turning to the following drawings wherein like elements are referenced by like numerals.